FIELD OF THE INVENTION
The invention relates to a sensitivity time control device in accordance with the preamble of the claim.
With imaging radar systems used today, radar pulses (k) are transmitted by means of an antenna 10, by means of which the backscatter signals e(k) are then received and forwarded to a transmitter-receiver unit 11, where they are down-mixed, as illustrated in the top part of a block diagram in FIG. 1. The Amplitude of the backscatter signals received is changed by means of two attenuators switched in series in the form of an automatic gain control attenuator (AGC) 12 and a sensitivity time control attenuator 13 (or STC unit 13).
A signal detector 14 is placed downstream of the sensitivity time control attenuator (STC) 13 for demodulation and detection. The output signal of the detector 14 is digitally converted in an analog-digital (A/D) converter 15 and forwarded via a formatting unit 16 to a recording unit 17.
Because the backscatter signal e(k) received by means of the antenna 10 can be large with range and may be, for example, up to 50 dB, the dynamic range of the imaging radar system must be correspondingly adapted. As a rule, however, the dynamic range of such a radar system is limited by the analog-digital conversion which has been performed in the converter 15. But without a sensitivity time control by means of the STC attenuator 13, large distortions would occur in the course of analog-digital conversion or corresponding quantization. In this case, the distortions in the course of quantization are the result of the sum of the so-called quantization noise and the saturation noise.
Because there is little or no information available regarding the terrain properties to be represented, it is a disadvantage of the known sensitivity time control device that it is not possible to determine the sensitivity time control curve exactly in advance. The analog-digital converter also cannot be optimally controlled for this reason. With the known time control devices it is necessary to calculate a fresh sensitivity time control curve for each flight geometry or for each system configuration. Because of this, particularly large expenditures are required for the operational use of the imaging radar system over a terrain, the backscatter properties of which are still unknown.
None of the existing sensitivity time control devices evaluates the backscatter signal in real time. Therefore the sensitivity time control curve is either determined in advance, if that is possible, or it must be manually set during the operation. This has been described, for example, in a publication in connection with a CCRS symposium in Canada in 1988 as special issue 88 CH 2572-6/88/0000-0015 of IEEE.